60-foot waves exploded off the Pacific Coast during October’s bomb cyclone

LOS ANGELES — The bomb cyclone and atmospheric river that pummeled Northern California in late October produced exceptionally heavy rain and high winds. But it also battered the California coast with some epic ocean waves. During the storm, peak individual wave heights of as much as 60 feet were measured from Washington to California, according to researchers from the Scripps Institution of Oceanography in San Diego.

For example, the No. 29 Point Reyes buoy, located in 1,805 feet of water 25 miles west of Point Reyes, recorded a significant wave height of 30.6 feet on Oct. 25. That’s the second-largest wave event in that buoy’s 23 years of recording data. Only in December 2015, an El Niño year, did it record bigger waves.

Significant wave height is calculated by averaging the height of the biggest one-third of waves during a 30-minute period, according to James Behrens, a program manager at the Coastal Data Information Program. Typically, some individual waves at a given station can jack up to as much as twice that average, and the Point Reyes buoy recorded a maximum individual wave height of 50.5 feet.

To the north, the No. 179 buoy off Astoria, Oregon, recorded significant wave heights of 35 feet, with individual waves slightly over 60 feet. This set a record for the station, which came online in 2011.

Later, as the storm weakened and the front sagged down the California coast, the No. 71 Harvest buoy, in 1,791 feet of water west of Point Conception, recorded a significant wave height of almost 30 feet, with a maximum individual wave height of 50 feet.

The deep low-pressure system that generated these historically large and powerful waves was churning off the Washington coast. It was part of a series of storms and atmospheric rivers that hit the West Coast in quick succession from Oct. 19 to 24. It had undergone explosive intensification called bomb-cyclogenesis, which means its central pressure dropped at least 24 millibars (a measure of pressure) in 24 hours. Generally speaking, the lower the atmospheric pressure, the more intense the storm.

Mid-latitude or extratropical cyclones such as this are low-pressure systems that generally occur between 30 degrees and 60 degrees latitude in the Northern Hemisphere.

This was the second bomb cyclone to develop in this part of the eastern Pacific Ocean in a few days. When its central pressure dipped to 942.5 millibars on the morning of Sunday, Oct. 24, it set a record for storms in this part of the ocean off the U.S. Pacific Northwest, and was equivalent to a Category 4 hurricane. The Saffir-simpson scale classifies hurricanes based on wind speed and central pressure. At the time, the storm was about 345 miles west of Aberdeen, Washington, and its winds were raking Northern California and the Pacific Northwest.